Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-18.co.uk. NS
Answer
awsdns-18.co.uk. 5 NS g-ns-1814.awsdns-18.co.uk.
awsdns-18.co.uk. 5 NS g-ns-1493.awsdns-18.co.uk.
awsdns-18.co.uk. 5 NS g-ns-338.awsdns-18.co.uk.
awsdns-18.co.uk. 5 NS g-ns-914.awsdns-18.co.uk.
Authority
The response has no authority records.
Additional
g-ns-338.awsdns-18.co.uk. 5 A 205.251.193.82
g-ns-914.awsdns-18.co.uk. 5 A 205.251.195.146
g-ns-1493.awsdns-18.co.uk. 5 A 205.251.197.213
g-ns-1814.awsdns-18.co.uk. 5 A 205.251.199.22
g-ns-338.awsdns-18.co.uk. 5 AAAA 2600:9000:5301:5200:0:0:0:1
g-ns-914.awsdns-18.co.uk. 5 AAAA 2600:9000:5303:9200:0:0:0:1
g-ns-1493.awsdns-18.co.uk. 5 AAAA 2600:9000:5305:d500:0:0:0:1
g-ns-1814.awsdns-18.co.uk. 5 AAAA 2600:9000:5307:1600:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0